Add Char compatibility module for 4.02.3

This is needed for configurator

src/stdune/caml/caml.ml

@@ -1,5 +1,6 @@
 module Filename = Filename
 module String   = String
+module Char     = Char
 module Result   = Result
 
 type ('a, 'error) result = ('a, 'error) Result.t =

src/stdune/char.ml

@@ -0,0 +1,9 @@
+module Char = Caml.Char
+
+include struct
+  [@@@warning "-32-3"]
+  let uppercase_ascii    = Char.uppercase
+  let lowercase_ascii    = Char.lowercase
+end
+
+include Char